Well, the comment blowing out a case has to do with wildcatting an existing case to increase the powder volume.

All cases have some taper to their body sections. This is needed so that the case will easily pop loose from the chamber after firing. The less taper you have the more difficult extraction can be so there is a certain point where we have to stop.

Most modern case designs, at least the ones in the last 30 years are minimal in taper, there are exceptions.

The rounds with the most taper such as the 22-250, 257 Roberts and for the big guns the 338 Lapua gain the most volume when their body taper is reduced and the shoulder diameter increased.

Rounds such as the 308 family and WSM rounds do not gain as much percentage wise.

TO do this a chambering reamer need to be built to the specs you want and your chamber then needs to be recut to these new dimensions.

Then, in most cases a standard parent case can be fired in this new fatter chamber and the pressure of the round will blow the parent case out to fit the larger dimensions of the new chamber.

Care should be taken though as some case modifications require special steps to insure safe headspace gas seals if the shoulder is set back any amount.

My two target guns require fire forming exsisting shells. For non standard wildcat rounds you can then send in a shot case and RCBS will make you the dies.

270 ackley improved actually only gained about 4-5 grains of powder over the standard 270 winchester.

Fire form - You just insert an off the store shelf round and pull the trigger. The pressure expands the case to fit the barrel dimensions. The improved round has a steeper shoulder and a longer body

I had a 223-7mm Tompsen Contender pistol. I bought cases of .223 rifle rounds and you pull the trigger. The .223 just bounces down the barrel. but the case opens up to the 7mm size. Really a nice gun with 130 gr. rounds